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Sharpening up JupiterBY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: October 02, 2008

The sharpest ever view of Jupiter has been achieved by using a superior technique to remove atmospheric blur, revealing changes in Jupiter's smog-like haze as part of the planet's so-called global upheaval.

The series of 265 snapshots were obtained with the prototype Multi-Conjugate Adaptive Optics Demonstrator (MAD) mounted on ESO's Very Large Telescope (VLT). The instrument corrects for atmospheric distortions, which normally give stars their twinkle as well as smearing out the finer details on a planetary surface. The technique uses two or more guide stars instead of the usual one as references to remove the blur over a field of view thirty times larger than existing techniques, resulting in an image as sharp as it would appear if you were observing in space.

"This type of adaptive optics has a big advantage for looking at large objects, such as planets, star clusters or nebulae," says lead researcher Franck Marchis from UC Berkeley and the SETI Institute in Mountain View, California. "While regular adaptive optics provides excellent correction in a small field of view, MAD provides good correction over a larger area of sky. And in fact, were it not for MAD, we would not have been able to perform these amazing observations."

This false colour photo is the combination of a series of images taken over a time span of about 20 minutes, through three different filters (2, 2.14, and 2.16 microns). The image sharpening obtained is about 90 milli-arcseconds across the whole planetary disc, a real record on similar images taken from the ground. This corresponds to seeing details about 300 km wide on the surface of the giant planet. The observations were done at infrared wavelengths where absorption due to hydrogen and methane is strong. This explains why the colours are different from how we usually see Jupiter in visible-light. This absorption means that light can be reflected back only from high-altitude hazes, and not from deeper clouds. Credit: ESO/F. Marchis, M. Wong, E. Marchetti, P. Amico, S. Tordo.

MAD allowed the researchers to observe Jupiter for almost two hours, a record duration, since conventional adaptive optics using a single Jupiter moon as reference is limited by the motion of the moon away from the planet. Neither can the Hubble Space Telescope observe Jupiter continuously for more than about 50 minutes, because its view is regularly blocked by the Earth during its 96 minute orbit.

Using MAD, ESO astronomers tracked Europa and Io on either side of Jupiter to provide a good correction across the full disc of the planet. "It was the most challenging observation we performed with MAD, because we had to track with high accuracy two moons moving at different speeds, while simultaneously chasing Jupiter," says MAD project manager Enrico Marchetti.

With this unique series of images, the team found a major alteration in the brightness of the equatorial haze that lies in a 16,000 kilometre wide belt over Jupiter's equator, when compared with images taken in 2005 by the Hubble Space Telescope. The Hubble images, taken at infrared wavelengths very close to those used for the VLT study, show more haze in the northern half of the bright Equatorial Zone, while the 2008 VLT images show a clear shift to the south. In addition, more sunlight reflecting off upper atmospheric haze means that the amount of haze has increased, or that it has moved up to higher altitudes.

"The brightest portion had shifted south by more than 6,000 kilometres," says team member Mike Wong "The change we see in the haze could be related to big changes in cloud patterns associated with last year's planet-wide upheaval, but we need to look at more data to narrow down precisely when the changes occurred."

Rare storms captured by Hubble in 2007 and earlier this year, showed towering storms punching through the top of the cloud deck in response to some global mechanism that has also seen Jupiter’s stripes and spots change colour.

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